The present invention relates to controllable attenuators and attenuation systems for attenuating optical energy transmitted through a fiber optic.
There is often a requirement in fiber optic systems for precise control of optical signal levels entering various system components. This is particularly true for systems at test and characterization stages of deployment. A controllable optical attenuator can be used, for example, to characterize and optimize the optoelectronic response of high-speed photoreceivers, wherein the detection responsivity is dependent on the average optical power incident on the photodiode.
The majority of controllable fiber optic attenuators currently commercially available rely on thin-film absorption filters. This requires breaking the fiber and placing the filters in-line. Controllable attenuation is then achieved by mechanical means such as rotating or sliding the filter to change the optical path length within the absorptive material. This adversely impacts the response speed of the device, the overall mechanical stability, zero attenuation insertion loss and optical back reflection. In general, broken fiber designs suffer numerous disadvantages such as high insertion loss, significant back reflection, and large size. These factors can be minimized, although such corrective measures typically result in added cost and/or size.
What is required are improved controllable fiber optic attenuators and attenuation systems which keep the optical fiber core intact and which achieve controllable attenuation via control of radiative loss from the fiber.
The present invention relates to controllable fiber optic attenuators (e.g., variable optical attenuators xe2x80x9cVOAsxe2x80x9d) and attenuation systems, designed to operate in the conventional telecommunication spectral windows of 1300 nm and 1550 nm, or any other wavelengths of interest, especially those at which single mode propagation occurs. The devices can be placed in fiber optic networks or systems by simple fusion splicing or connectorization to attenuate optical signal levels by a desired amount. Controllable attenuation is achieved, for example, by thermal or electrical control of controllable material layers. The devices can be used for controllable attenuation in fiber optic systems at the test and characterization stage, or for active control during operational deployment.
The side-polished fiber (xe2x80x9cSPFxe2x80x9d) devices of the present invention are an improvement over conventional broken fiber approaches because of their intrinsic fiber continuity.
In a first embodiment of a controllable attenuator of the present invention, a fiber is mounted in a block and polished to within a close proximity (e.g., a few microns) of the core. A controllable bulk material, with an approximately matched refractive index (to the effective fiber mode index) is applied over the polished surface. Adjusting the index of refraction of the bulk material (e.g., via the electro- or thermo-optic effect), results in a controllable amount of optical energy extracted from the fiber optic, thus achieving controllable attenuation.
An attenuation system, including a controllable attenuator, is also disclosed in which a control circuit applies a changeable stimulus to the controllable material, in accordance with a desired level stimulus, and/or a sensed level stimulus received from a sense circuit coupled to the fiber optic for sensing a level of optical energy being transmitted therein.
In an improved embodiment of the controllable attenuator of the present invention, the fiber is polished through its cladding almost to its core, and a thin controllable material is placed between the fiber and a high-index, bulk overlay material. The index of refraction of the controllable material (approximately matched to that of the cladding) is varied, which effectively varies the effective optical thickness (index x actual thickness) of the remaining cladding. This improved, cladding-driven (xe2x80x9cCDxe2x80x9d) controllable attenuator provides nearly spectrally flat optical attenuation in the wavelength ranges of interest, while retaining all of the intrinsic advantages of the SPF architecture. Moreover, a design is disclosed where the typically used radius block holding the fiber is eliminated, which allows the device to be reduced in size so that it is not much larger than the fiber itself.
In that regard, the present invention relates to, in its first embodiment, an attenuation system for attenuating optical energy being transmitted through a fiber optic. A controllable attenuator is arranged with respect to a portion of the fiber optic having material removed therefrom thereby exposing a surface thereof through which at least some of the optical energy can be controllably extracted. The attenuator includes a controllable material formed over the surface for controllably extracting the optical energy according to a changeable stimulus applied thereto which affects the refractive index thereof. A level sensing circuit may be coupled to the fiber optic for sensing a level of at least a portion of the optical energy transmitted therein and providing a sensed level stimulus to a control circuit, which is coupled to the controllable attenuator for applying the changeable stimulus to the controllable material thereof in accordance with the sensed level stimulus received from the level sensing circuit.
The changeable stimulus applied to the controllable material may be, for example, temperature (thermo-optic effect) or voltage (electro-optic effect).
In a second, improved aspect, the present invention relates to a cladding-driven (xe2x80x9cCDxe2x80x9d) controllable attenuator for attenuating optical energy transmitted through a fiber optic. The controllable attenuator is arranged with respect to a portion of the fiber optic having material removed therefrom thereby exposing a surface thereof through which at least some of the optical energy being transmitted therein can be extracted. The controllable attenuator includes a controllable material formed over the exposed surface for controlling an amount of optical energy extracted from the fiber optic according to a changeable stimulus applied to the controllable material which affects the index of refraction thereof. In addition, a bulk material layer formed over the controllable material is provided into which the extracted optical energy is radiated.
In this embodiment, the controllable material has a controllable index of refraction approximately matching the index of the cladding, and the bulk material formed over the controllable material has a fixed index of refraction higher than the effective mode index of the fiber optic.
The controllable fiber optic attenuators and attenuation systems of the present invention are valuable in any applications where control of the optical power transmission in an optical fiber is required. The attenuators are especially useful in applications where the spectral flatness of attenuation is a concern. Because of the fiber continuity, these devices exhibit the intrinsic benefits of low insertion loss, low back reflection (high return loss), polarization insensitivity, small size, low cost, and mass produceability.